1. Field of the Invention
This invention relates to a martensitic stainless steel pipe which has good strength and toughness, and is suitable for use as a material for drilling oil wells or natural gas wells, and constructing various plants and buildings.
2. Description of The Relates Art
Martensitic stainless steel represented by a 13% Cr martensitic stainless steel, is generally used in the quench hardening and tempering condition to improve strength and corrosion resistance. Since this type of steel pipe has very good hardenability, it can be well hardened to the center of a pipe wall, depending on the size and chemical composition thereof, even if air cooling from high temperature is applied. In case where quench hardening is carried out by use of a refrigerant, the usual practice is to employ oil cooling which permits a slow cooling rate.
However, a steel having good hardenability tends to suffer quench cracks or deformation by quenching. The hardening of such steel is ascribed to the transformation of the austenite phase at high temperatures into a martensite phase by quenching. This transformation brings about a great volumetric expansion. Accordingly, when the cooling rate is too high, heterrogenous, abrupt deformation takes place, resulting in the local concentration of internal stress, to cause cracks.
In recent years, it becomes necessary to drill oil or natural gas well under severe conditions of a corrosive environment. This, in turn, requires a steel pipe, having high corrosion-resistant and high strength for use as oil well tubular goods or allied facilities. For the manufacture of such pipe, there have been developed direct quench hardening methods wherein a steel pipe under still high temperature condition, just after hot workings such as piercing, and rolling, is hardened as it is. However, in the manufacture of stainless steel pipes, having a martensite structure, cracks can occur due to rapid cooling, such as water cooling, as the direct quench hardening method, thus making it difficult to apply quench hardening in water. Thus, it inevitableiy takes a long time, to sufficiently cool slowly from high temperatures, presenting the problem that the productivity lowers considerably. Moreover, the cooling rate cannot be made great, so that a wide space for keeping steel pipes being cooled over a long time becomes necessary, inviting a rise in facility cost.
For a hardening method of 9% Cr or 13% Cr martensitic stainless steel, there is disclosed, in Japanese Laid-open Patent Application No. 3-82711, a method wherein a steel pipe, having a wall thickness of 10 to 30 mm is acceleratedly cooled at a rate of 1 to 20.degree. C./second by blowing water from a nozzle thereagainst. In water quenching, wherein a heated steel pipe is immersed into a water vessel, the quenching rate is 40.degree. C./second or over, resulting in quench cracks in most cases. If, however, the cooling rate is appropriately controlled, as a disclosed method, little or no quench crack takes place, with the attendant advantage that the cooling efficiently proceeds. However, when the above disclosed method is adopted, a particular cooling apparatus and control means are needed in addition to those for an ordinary carbon steel pipe. In addition, although the above method permits a high cooling rate, the rate is not greater than half of a cooling rate in the water immersing method,so that a remarkable improvement in productivity can not be achieved.